Crypto: Rename Invisible Crypto lab flag

[BillCarsonFr]

Follow up on matrix-org/matrix-js-sdk#4428
Part of element-hq/element-meta#2536

CryptoMode / Invisible Crypto isn't very descriptive.

We want to switch to DeviceIsolationMode, so the config flag should be name Enable OnlySigned Device isolation mode (description could be Ignore unverified devices when sending/receiving messages)

[richvdh]

Follow up on matrix-org/matrix-js-sdk#4428

wrong link, I think? EX iOS does not use the js-sdk.

[mxandreas]

Ignore unverified devices when sending/receiving messages

The new terminology suggest using only "not secure" devices (and specifically not using any other similar terms). I think we should be role models in following that terminology? I also see that we have not adopted this new terminology in many other parts of our work - tickets, documents, etc.

Adding one editorial note, I would suggest: Exclude not secure devices when sending/receiving messages